The following abbreviations are herewith defined:                3GPP Third Generation Partnership Project        AMC Adaptive modulation and coding        DL downlink (Node B to UE)        H-ARQ hybrid automatic repeat request        HSPA high speed packet access        IP internet protocol        L1 Layer 1 (Physical (PHY) Layer)        L2 Layer 2 (Link Layer)        LTE Long Term Evolution        MAC medium access control        Node B base station        OFDMA orthogonal frequency division multiple access        PDU protocol data unit        QoS quality of service        RF radio frequency        SC-FDMA single carrier-frequency division multiple access        SDU service data units        TB transport block        TTI transmission time interval        UE user equipment        UL uplink (UE to Node B)        UMTS Universal Mobile Telecommunications System        UTRA Universal Terrestrial Radio Access        UTRAN Universal Terrestrial Radio Access Network        E-UTRAN Evolved UTRAN        
In advanced cellular networks, such as UTRA LTE, when IP packets are transmitted over the radio interface using these different spectrum allocations, the link layer (L2) of the radio interface, including the MAC functionality, can segment IP-based SDUs passed down by an upper layer into one or several segments and, at the same time, pack one or multiple segments into a PDU for further physical layer (L1) transmission. These two processes, L2 SDU segmentation and L2 PDU packing are both needed to ensure robust transmissions of IP packets with variable packet sizes in bits or bytes over radio channels with variable bit rates. However, these segmentation and packing processes typically result in an increased overhead in the processing of IP packets. This introduces the need for efficient packet header mechanisms to minimize the overhead in processing the IP packets.